Stag Lane (Edgware) Observatory
Full-disk solar H alpha imaging using William Optics and Lunt setup
2010 March
I have been taking images of the Sun in recent months in hydrogen alpha with a Lunt LS60T hydrogen alpha telescope, double-stacked, with the B1200 blocking filter, and either a Lumenera SkyNYX 2-0 camera, or a DMK 21AF04.AS camera. However, I have been experimenting to find a setup to allow me to image the whole Sun in one go in H alpha, and have come up with a solution that seems to produce decent results. The Lunt LS60T H alpha telescope does not really work for this, particularly when double stacked, as the filtering is too uneven across the focal plane (the system has a pronounced "hot band") and the focal length is anyway too long to get the whole Sun on any high frame-rate camera chip that I know of.
The new solution is a Willam Optics ZS66 refractor double-stacked with two 50mm Lunt front etalons, and the B1200 filter at the back end in front of a DMK 41AF02.AS camera. This has a half-inch CCD 1280x960, and does a maximum of 15 fps. At the FL of the ZS66, 388mm, the whole Sun fits comfortably on the chip. I had to do a certain amount of "adapting" to screw the DMK, with its C-mount thread, to the B1200 filter, and the other side of the filter to the ZS66 in such a way that the system reached focus. The system is shown below, mounted on my AP900, with a C-11 and 100mm refractor there as well.
William Optics ZS66 double-stacked for solar H alpha imaging, on AP900GTO mount
To link the camera to the B1200 diagonal, the eyepiece receiver tube of the diagonal was unscrewed, revealing a male T thread. A locking T-ring was screwed onto this and a C-mount to T adaptor above this. The camera threads on to this adaptor. The locking ring allows the camera to be rotated and locked at the correct orthogonal position. To link the B1200 diagonal to the ZS66 was quite difficult, as the ZS66 uses a 2" SCT male thread on the drawtube. The solution was to screw a Meade 2" thread to T adaptor (the one that comes with the largely obsolete f3.3 focal reducer) to the drawtube, and remove the 1.25" nosepiece on the B1200 diagonal. This results in two male T-threads needing to mate. They were mated using another T locking ring and an internally T-threaded ring that was part of some camera adaptor once (I have never encountered this component on sale separately).
Details of camera-filter-telescope assembly
I have found that by adjusting the tuning of both the front-end filters with this system I can obtain a fairly even illumination of the whole disk with a reasonably narrow bandpass. The system does have a "hot band" when optimally tuned visually, but tuning away from this evens the illumination and still gives enough light and contrast for the camera.
Below are whole-disk images from the March 4th and 7th taken with this system and processed to show something of the prominences as well as the surface, from single videos. Seeing was poor on both occasions, with the Sun almost setting for the full-disk image on the 4th. Further images should become available on the solar pages of my astro-imaging catalogue, hopefully documenting a revival in the solar cycle, compared to these fairly bland images.
